Because of their mechanical and physical properties, such as higher stiffness, corrosion and wear resistance and greater thermal stability, ceramics such as silicon nitride (Si3N4) ceramics are considered the materials of choice to replace steels in such applications as contact rolling elements, e.g. bearings, where stiffness and wear resistance play a key role. There is also interest in these types of ceramics for high-temperature turbine bearing components where increased thermal stability is critical. For these types of applications, the most critical portions of the ceramic component, i.e., those with the highest stress during operation, are the surface or near-surface (usually to depth of <200 microns) regions. The most common types of defects found in these critical regions are mechanical in nature, such as cracks, spalls, inclusions, voids, etc., and can be either machining or operation induced.
During machining of ceramics, the material encounters high stresses and temperatures. This can result in the formation of radial, lateral and longitudinal cracks. Usually radial and lateral cracks do not significantly reduce the strength of the ceramic. The longitudinal, also called median, cracks are thought to cause the greatest reduction in strength.
Ceramic components are increasingly being studied for use in gas turbines for rotating bearings, vanes and blades. In bearing applications, the critical regions of a component experiencing the highest stresses are frequently the surface and the near-surface regions to a depth on the order of 200–300 microns. A similar argument can be made for components in bending stress applications.
Some of the most critical defects are thus located on or just beneath the surface and originate when the manufactured part undergoes machining. Any machining induced damage which causes part rejection is to be avoided as early in the manufacturing process as possible to avoid cost-added to rejectable parts. An on-line method for determining the amount of surface and sub-surface damage imparted to a ceramic thus has an economic benefit. Using an on-line detection method, machine tool feed rates and contact pressures can be optimized during machining to obtain the highest material removal rates without adversely affecting the mechanical or tribological properties of the ceramic.
One non-destructive method for detecting and analyzing the defects in ceramics employs polarized laser light directed onto the surface of the ceramic body. A simplified schematic diagram of an optical scattering detection system 10 for use in inspecting ceramic materials is shown in FIG. 1. In this arrangement, a polarized laser beam 14 is directed onto the surface of an object 12 being studied. The incident light is polarized to allow for discrimination between surface and subsurface defects using the material's Brewster angle. A portion of the incident laser beam 14 is directed through the material as a transmitted beam 18 and a portion is reflected as well as scattered from the surface of the object 12 in the form of a surface reflected beam 16. A portion of the incident laser beam 14 also appears as internal scatter 20 within the object 12 being studied and is absorbed by the object. The incident laser beam 14 must be normal to the surface of the object 12 and the detected reflected beam 22 is directed through a polarized object lens 24 and to a first detector 26 as well as to a second detector 28. The first detector 26 detects light reflected from the surface of the object 12, while the second, larger detector 28 detects light scattered by the subsurface portion of the object. Graphically shown in FIG. 1a is the distribution of back-scattered light from the object under investigation, with increased back-scattering occurring in the presence of a defect in the object under investigation. The intensity of the back-scattered light is plotted along the vertical axis, while the spatial distribution of the back-scattered light is plotted along the horizontal axis.
Prior approaches in the area of non-destructive evaluation and characterization of ceramic and ceramic coated objects in a production environment have met with only limited success. For example, because the incident light must be perpendicular to the surface of the object under investigation and because most objects are of an irregular shape, prior approaches involved complex movement of the components under investigation and/or the incident laser beam. Complicated movement of the component under investigation requires a sophisticated displacement arrangement making measurement stability and repeatability more difficult. In addition, prior complicated ceramic component and/or laser beam displacement and positioning systems require an equally complex optical arrangement increasing the time required for detection and analysis of defects rendering these prior approaches impractical for use in on-line production inspection of ceramic components.
The present invention addresses the aforementioned limitations of the prior art by providing optical apparatus for use in detecting and analyzing defects and micro-structural changes in monolithic and composite structural ceramic components as well as in deposited coatings. The optical apparatus if particularly adapted for use with irregularly shaped components and provides highly accurate and repeatable nondestructive evaluation of objects having complex shapes in realtime so as to be particularly effective in an on-line production inspection environment.